The present disclosure relates to an electromagnetic servo brake comprising a master cylinder primary piston thrust in the direction of actuation thereof by a thrust rod, which is in turn pressed against the servo piston.
In electromagnetic servo brakes, the primary piston is thrust by a thrust rod identical to that of the vacuum servo brake, in which the thrust rod is formed by a rod so to speak connected to a base in the form of a piston pressing by way of the reaction disk against the servo piston actuated by the pneumatically operating braking force amplifier or by the electric motor driving this piston by way of a rack transmission.
In the case of the servo piston connected to the pneumatic piston, the assembly is mounted so as to float in a relative manner and has sufficient freedom so as not to impose a component of transverse force on the primary piston, but solely a longitudinal force.
In the case of the electromagnetic servo brake, the servo piston is held rigidly by the connection thereof by rack to the electromagnetic drive, such that it is not adapted and the thrust exerted onto the primary piston has a relatively significant transverse component.
Document FR 03 11 580 describes an electrical brake assist servomotor shown in FIG. 1 of said document, which has been included as FIG. 1 of this patent.
FIG. 1 shows the servo brake, in section, combined with the tandem master cylinder 48, of which the primary piston 49 is thrust by the servo piston 20 by way of a thrust rod 47 or push rod. This push rod is pressed against the servo piston by way of a reaction disk 40. The thrust rod 47 is formed of two parts so as to make it possible to adapt the length of said thrust rod to the specific type of servo brake; the thrust rod has a rounded front end and a rear end in the form of a disk pressed against the reaction disk. The thrust rod 47 is thus blocked in the direction of the axis (xx) of the servo brake, with no degree of freedom, such that the transverse component of the thrust exerted by the servo piston 20 onto the thrust rod 47 is translated by a transverse component of force exerted onto the primary piston 49, which causes uneven wear thereof.
The references used in the description of the present FIG. 1 are those from document FR 03 11 580. The references not described and shown in FIG. 1 relate to the elements of this known servo brake not described here since they are not directly involved in the presentation of the present disclosure.
By way of comparison, tests have shown that under the same operating conditions, the transverse component exerted by the thrust rod of a pneumatic servo brake onto the primary piston was negligible, whereas in the case of an electromagnetic servo brake this component was significant.